In the injection molding art, it is desirable to quantitatively measure the performance and capability of both the injection molding process and the product produced therefrom. The techniques currently used for this purpose are commonly referred to as "process capability indices". These indices are less than ideal because they are based on two false assumptions: (1) that the process/product characteristics are statistically symmetrical and (2) the quality of all products within the established range of specifications are the same, even though some are closer to ideal dimensions than others.
A more meaningful characterization of the performance of an injection molding process could be achieved in a method which takes account of the realities concerning the process, i.e., that product characteristics are not statistically symmetrical and that the quality or value of a product which is within specifications decreases as its dimensions vary from ideal dimensions.
Those skilled in the art are undoubtedly familiar with the Taguchi loss function concept. According to this concept, a product within an acceptable range of dimensional specifications is considered to be most valuable if its dimensions match the ideal dimensions for that product. As a dimension varies from the ideal dimension, its value decreases quadratically as it reaches specification tolerance limits. The Taguchi concept is applied and extended to the injection molding process according to the subject invention by treating data from parts produced during a standard process robustness certification procedure as a "designed experiment". Those familiar with the art of injection molding know that it is customary to certify that a given process is sufficiently robust. This certification is accomplished through the use of a molding area diagram in which the molding process is performed at a selected number of data points at the extremes and midpoints of temperature and pressure for that process.
Each part characteristic or measurement can be quantitatively partitioned into the four injection molding process elements affecting manufacturing capability: (1) target error; (2) cavity bias; (3) process error; and (4) other.
It is, therefore, an object of this invention to provide an improved method for characterizing the performance of an injection molding process.
It is a further object of this invention to provide a method of the type described which takes account of the statistical asymmetric nature of injection molding data.
It is a further object of this invention to provide a method of the type described which takes advantage of the Taguchi loss function concept.
It is a further object of this invention to provide a method of the type described which can be applied to existing robustness certification methods.
It is a further object of this invention to provide a method of the type described which results in the identification of the process error elements most in need of correction.